1. Field of the Invention
The present invention relates to a method for preparing a solid film lubricant with superior lubricity for parts of machinery that operate under conditions of extremely high stress.
2. Description of the Related Art
Solid film lubricants are necessary for long-lasting lubrication of machinery parts, dry lubrication under special conditions such as high-temperature and high-vacuum conditions, and for material surfaces having particular functional properties such as improved abrasion resistance and lowered friction of the surfaces coated with the lubricants, etc. In recent years, interests in solid film lubricant related technology are growing in the mechanical and electronic industries with wide applications for office machinery, vehicles, vacuum apparatuses, food processing apparatuses, precision apparatuses, printing machines, aerospace related apparatuses, chemical related equipment, etc.
As an example, when a bearing is operated under high-load, high-temperature, or vacuum conditions, a lubricant film coated on the bearing surface is damaged, causing galling by heat generated by direct contact between metals. In particular, common aqueous lubricants are inadequate to provide lubrication in a vacuum environment or at extremely low or high temperatures. In addition, when operation of a piece of liquid lubricated machinery is halted, the aqueous lubricant undesirably flows down off of the surfaces where it is needed due to gravity, thereby causing trouble and potential damage when restarting the machinery.
To avoid such problems, in prior art, a coating mixture of a lubricant material, such as graphite, molybdenum disulfide (MoS2), tungsten disulfide (WS2), polytetrafluoroethylene (PTFE), or boric nitride (BN), and a binding agent such as an organic or inorganic compound is used. A solid film lubricant is formed on a pretreated target part by spraying in air or by sputtering of the coating mixture in a vacuum chamber. U.S. Pat. No. 4,473,381 discloses a lubricant film which is capable of effectively preventing galling of sliding metallic parts. The lubricant film comprises 60-80% by weight molybdenum disulfide or molybdenum disulfide and graphite, 10-30% by weight additive for improved thermal stability and anti-oxidation, such as antimony oxide (Sb2O3), iron (Fe) powder, zinc (Zn) powder, or gold (Au) powder, and an organic binder such as epoxy-ester resin, acrylic resin or urea resin. Other molybdenum disulfide-resin based lubricants are disclosed in U.S. Pat. Nos. 3,051,586; 4,303,537; 3,146,142; and 4,206,060.
Japanese Patent Publication No. hei 4-26777 discloses a method for forming a solid film lubricant with improved thermal resistance and lubrication on a titanium (Ti) or titanium alloy plate. Prior to deposition of the solid film lubricant on the surface of a base metal, heating at 500xc2x0 C. in a vacuum and chemical activation are carried out on the base metal as pre-treatments to attain a porous surface. The base metal with the porous surface is electroplated with a composite material such as nickel-phosphate or silicon-carbide, thereby improving thermal resistance and durability. Japanese Patent Publication No. sho 61-4797 discloses a method of coating a molybdenum-epoxy resin based film lubricant on the surface of a metal part by thin spraying of a dispersion of molybdenum disulfide and epoxy resin in a solvent after degreasing the surface of the metal part.
However, for the solid film lubricant formation using the coating material including organic or inorganic binders described above, it is difficult to accurately control a ratio of the solid lubricant and the binder as well as the thickness of the solid film lubricant, thereby leading to a problem of non-uniform thickness, which can be serious for precision parts. In addition, additional post-processes, such as lapping of the solid film lubricant, need to be performed. In this case, aside from difficulties in processing, there is a limitation in lapping the thickness of the remaining solid film lubricant to an appropriate level.
Meanwhile, a pigment used as a lubricant in the solid film lubricant has a great specific gravity and oil absorbency so that it is very likely to settle down if it is used in the form of a coating mixture along with a resin. Therefore, it is difficult to determine an optimal ratio of the pigment serving as a lubricant and the resin serving as a binder. The greater the content of resin in the solid film lubricant, the worse the lubricity. The greater the content of pigment, the better the lubricity, but the worse the durability. To make up for the drawbacks of the solid film lubricant formed by spraying, a method for forming a solid film lubricant by sputtering, which is a kind of dry coating, has been suggested and applied for a variety of parts commonly used in the aerospace, defense, and high-precision industries. The processing costs are very high due to costly equipment used for the sputtering and slow sputtering rate. Therefore, the solid film lubricant formation by sputtering cannot be applied to commonly used parts.
It is an object of the present invention to provide a method for forming a long-lasting solid film lubricant with uniform thickness, which can be applied to parts of any shape without using resin as a binding agent.
To achieve the object of the present invention, there is provided a method for forming a solid film lubricant on the surface of a part having an arbitrary shape, the method comprising: preparing a carrier having a predetermined shape and size; coating lubricant powder on the carrier; and coating the lubricant powder over the surface of the part by physical contact between the carrier coated with the lubricant powder and the part.
It is preferable that the carrier coated with the lubricant powder and the part are made to contact physically each other by application of mechanical force selected from the group consisting of rotational, vibrational, frictional and impulsive forces. It is preferable that the lubricant power comprises at least one selected from the group consisting of molybdenum disulfide (MoS2), tungsten disulfide (WS2), graphite, boric nitride (BN), and polytetrafluoroethylene (PTFE). It is preferable that, in coating lubricant powder on the carrier, an organic or an inorganic binder is coated along with the lubricant powder on the carrier.
It is preferable that the solid film lubricant formation method further comprises forming at least one intermediate layer on the surface of the part for improved adhesion to the lubricant powder, before causing contact between the carrier coated with the lubricant powder and the part. It is preferable that the intermediate layer is formed of at least one material selected from the group consisting of silver, copper, tin, lead, gold, zinc, cadmium, an alloy of these metals, and a composite alloy of a solid lubricant and these metals. It is preferable that the intermediate layer is formed by at least one method selected from the group consisting of electroplating, chemical plating, vacuum plating, thermal spraying, and physicochemical deposition. The method preferably further comprises heating the part with the intermediate layer at a temperature of 150-500xc2x0 C. for improved ductility of the intermediate layer, after forming the intermediate layer.